An interconnect circuit board of an LTCC design is a physical realization of electronic circuits or subsystems made from a number of extremely small circuit elements that are electrically and mechanically interconnected. It is frequently desirable to combine these diverse electronic components in an arrangement so that they can be physically isolated and mounted adjacent to one another in a single compact package and electrically connected to each other and/or to common connections extending from the package.
Complex electronic circuits generally require that the circuit be constructed of several layers of conductors separated by insulating dielectric layers. The conductive layers are interconnected between levels by electrically conductive pathways, called vias, through a dielectric layer. The conductors useful in LTCC technology are typically thick film conductors. LTCC multilayer structures allow a circuit to be more compact than traditional Al2O3 substrates by allowing vertical integration.
Similar to other thick film materials, thick film conductors are comprised of an active (conductive) metal and inorganic binders, both of which are in finely divided form and are dispersed in an organic vehicle. The conductive phase is ordinarily gold, palladium, silver, platinum or alloy thereof, the choice of which depends upon the particular performance characteristics which are sought, e.g., resistivity, solderability, solder leach resistance, bondability, adhesion, migration resistance and the like. In multilayer LTCC devices, additional performance characteristics are sought for internal conductor lines and via conductors which include minimization of conductor line “sinking” into the top and bottom dielectric layers on firing, minimal resistivity variation on repeated firing, optimal interface connectivity of line conductor to that of via-fill conductor, and optimal interface bonding of via-fill conductor to that of surrounding ceramic materials.
LTCC devices have been used in prior art high frequency applications, such as telecommunications, automotive or military applications including radar, for its multilayer, cofiring and flexible design capabilities. Many properties are required of the conductors used in the fabrication of multilayer circuits for high frequency including microwave applications, including desired resistivity, solderability, solder leach resistance, wire bondability, adhesion, migration resistance, and long term stability.
In addition to the proper level of conductivity and other properties listed above, there are many secondary properties which must also be present such as, wire bondability, good adhesion to both ceramic and thick films, solderability and compatibility to other thick films, both surface and buried, long-term stability without little or less properties degradation.
The use of precious metals such as gold conductors increases the cost of high-reliability LTCC devices and manufacturers, as would be expected, look for ways to reduce cost by reducing the use of such precious metals. One such method is to employ silver-based conductors rather than gold conductors. The reliability of silver-based conductors is relatively lower and wire-bonding is not possible. Gold conductors and Gold via fills may still be used in strategic placements to mitigate the effects of the lower cost, lower performance silver-based materials.
A conductive system for use in LTCC conductive elements which uses Pd/Ag or Pd/Pt/Ag mixtures is disclosed in commonly assigned U.S. Pat. No. 7,611,645 to Nair et al. A palladium-silver transition via fill conductor for a mixed metal system is disclosed in commonly assigned U.S. Pat. No. 7,550,319 to Wang et al.
However, such systems may be disposed to certain phenomena at the surface interface(s) between dissimilar metals. For example, the Kirkendall Effect, has been found in various alloy systems and is important in connection with bonding between different materials. In particular, it has been studied and is used to describe voids that are produced in the boundary region at a bonding interface especially during high temperature processes such as metallic powder sintering.
It is also considered that there may be EMF difference generated defects at the interface between different metals such as at an electrical connection in an LTCC device.
It would be desirable to employ a system which minimizes or eliminates such surface defects at bonds or connections between different metal materials in electrical devices such as LTCC devices